The most dramatic difference between rod and cone photoreceptors is their ability to adapt to background illumination. In mammals, rods hardly adapt at all and they do to some limited extent in non-mammalian species. In contrast, cones in all vertebrate species adapt to background lights that change over a range of about 10 log units of absolute intensity. The long-term objective of this research project is to uncover the molecular mechanisms responsible for light-adaptation in cone photoreceptors. It was recently demonstrated that this most significant difference between rods and cones reflects the function of a yet to be identified protein that modulates the sensitivity to cGMP of the cGMP-gated channels of cones in a Ca2+-dependent manner. This function simply does not exist in rods. To continue this investigation, the cone channel modulator must be identified and characterized. This objective will be followed through molecular genetics approach with the following specific aims: Specific Aim 1. To identify the Ca2+ dependent modulator of the cyclic GMP-gated ion channels of cone photoreceptors. Yeast two-hybrid screening will be used to identify the modulator by its interaction with the cGMP-gated channel cytoplasmic domain. Specific Aim 2. To verify the physical interactions of the identified under Specific Aim I protein with the channel in vitro and the physiological relevance of the interaction in situ. In vitro binding assays, in situ hybridization and immunohistochemistry will be used to verify the physiological relevance of the interactions. Specific Aim 3. Test the functional effectiveness of cloned modulator. The functional characteristics of the cloned modulator will be tested in intact cones in electrophysiological experiments. If successful, discovery and analysis of this previously unrecognized protein would be of impact in the elucidation of a long-standing physiological enigma. The successful completion of this project will provide us for the first time with detailed understanding of a novel molecular mechanism of the Ca2+-dependent modulation of cGMP gated channel ligand sensitivity that is characteristic for cone photoreceptors and is not present in rods. Success will provide yet another target protein in the molecular analysis of photoreceptorrelated genetic diseases (retinitis pigmentosa, for example).